docs: def Duckler(x, mul, mug, rhol, rhog):
      r'''Calculates a suggested definition for liquid-gas two-phase flow 
      viscosity in internal pipe flow according to the form in [1]_ and shown
      in [2]_, [3]_, and [4]_.
          
      .. math::
          \mu_m = \frac{\frac{x\mu_g}{\rho_g} + \frac{(1-x)\mu_l}{\rho_l} }
          {\frac{x}{\rho_g} + \frac{(1-x)}{\rho_l}   }
          
      Parameters
      ----------
      x : float
          Quality of the gas-liquid flow, [-]
      mul : float
          Viscosity of liquid, [Pa*s]
      mug : float
          Viscosity of gas, [Pa*s]
      rhol : float
          Density of the liquid, [kg/m^3]
      rhog : float
          Density of the gas, [kg/m^3]
  
      Returns
      -------
      mu_lg : float
          Liquid-gas viscosity (**a suggested definition, potentially useful
          for empirical work only!**) [Pa*s]
  
      Notes
      -----
      This model converges to the liquid or gas viscosity as the quality
      approaches either limits.
      
      This has also been expressed in the following form:
          
      .. math::
          \mu_m = \rho_m \left[x\left(\frac{\mu_g}{\rho_g}\right) 
          + (1 - x)\left(\frac{\mu_l}{\rho_l}\right)\right]
          
      According to the homogeneous definition of two-phase density.
          
      Examples
      --------
      >>> Duckler(x=0.4, mul=1E-3, mug=1E-5, rhol=850, rhog=1.2)
      1.2092040385066917e-05
  
      References
      ----------
      .. [1] Fourar, M., and S. Bories. "Experimental Study of Air-Water 
         Two-Phase Flow through a Fracture (Narrow Channel)." International 
         Journal of Multiphase Flow 21, no. 4 (August 1, 1995): 621-37. 
         doi:10.1016/0301-9322(95)00005-I.
      .. [2] Awad, M. M., and Y. S. Muzychka. "Effective Property Models for 
         Homogeneous Two-Phase Flows." Experimental Thermal and Fluid Science 33,
         no. 1 (October 1, 2008): 106-13. 
      .. [3] Kim, Sung-Min, and Issam Mudawar. "Review of Databases and
         Predictive Methods for Pressure Drop in Adiabatic, Condensing and
         Boiling Mini/Micro-Channel Flows." International Journal of Heat and
         Mass Transfer 77 (October 2014): 74-97.
         doi:10.1016/j.ijheatmasstransfer.2014.04.035.
      .. [4] Aung, NZ, and T. Yuwono. "Evaluation of Mixture Viscosity Models in 
         the Prediction of Two-Phase Flow Pressure Drops." ASEAN Journal on 
         Science and Technology for Development 29, no. 2 (2012). 
      '''
      return (x*mug/rhog + (1. - x)*mul/rhol)/(x/rhog + (1. - x)/rhol)